1. Field of the Invention
The present invention relates to radar systems and in particular to the suppression of unwanted signals received by a radar system which have been reflected from fixed objects (clutter) such as terrain features and buildings, in the case when the radar is mounted on a moving platform such as a ship.
2. Discussion of Prior Art
A radar system operates by transmitting electromagnetic signals and receiving these same signals after they have been reflected from targets. However, besides the reflections from the wanted targets, e.g. aircraft, other reflections from unwanted objects occur. These unwanted objects are referred to as clutter, and may include terrain features such as hillsides and cliffs, man made objects such as buildings and fences, and weather phenomena such as rain. The unwanted reflections may in some cases be greater than the reflections from targets.
Methods for suppressing the unwanted reflections and retaining only signals from targets generally rely on the fact that the clutter objects are stationary or only slowly moving. Many radars employ Moving Target Indication (MTI) or Moving Target Detection (MTD), which rely on determining the radial component of the target's velocity (towards or away from the radar) by examining the Doppler shift of the received signal. Suppressing signals which do not have a significant Doppler shift ensures that only fast moving objects are shown as targets. However, this does not completely solve the problem because some targets may have zero radial speed even if they are moving rapidly in a direction tangential to the radar. These wanted targets exhibit low Doppler shifts and are therefore suppressed along with the clutter if only MTI or MTD methods are used.
An alternative approach, which complements MTI and MTD processes used in many radar systems, is to vary the sensitivity of the radar receiver so that areas which are found to return strong signals are examined with lower sensitivity than areas which do not. This involves using a clutter map, in which the radar coverage area is divided into cells and an array of background signal estimates for these cells is stored. Received signals are then only accepted as being from wanted targets if they exceed the stored background level for the cell they occupy by a sufficient factor: if s is the received signal strength and b.sub.i is the background level stored in the cell, a detection is reported if s &gt;k.sub.T b.sub.i, for some fixed ratio k.sub.T.
The background estimates throughout the map are modified on each scan of the radar so that they gradually converge to the signal levels actually being received. Reflections from fixed scatterers therefore `build-in` to the map and are eventually suppressed. Moving targets, on the other hand, are detected whenever their reflections exceed the levels stored in the clutter map cells they temporarily occupy. If the map cells are small enough, moving targets will not remain in the same cells long enough to `build-in` and affect the stored levels. The operation of clutter maps for stationary radars has been fully described in the literature by M I Skolnik, Radar Handbook, McGraw Hill Book Company, 1978.
The background estimate in each clutter map cell is normally accumulated by `.alpha. smoothing`. If b.sub.i, is the current clutter background level in the ith cell, and the greatest level seen in the cell during the current scan is g.sub.i, the following expression is used to compute the new background level to be used for the next scan period: EQU (1-.alpha.)b.sub.i +.alpha.g.sub.i
where .alpha. is a small number, typically 1/8 or 1/16.
The above description of clutter map operation has assumed that a clutter map has already been set up when radar data is received. It is also necessary to consider the problem of initialisation: filling the clutter map with suitable data when the radar is first switched on. If this is done incorrectly, it is possible for many false detections to be reported on the first few scans, due to low initial values of b.sub.i causing the condition s &gt;k.sub.T b.sub.i to be frequently met.
The operation of the clutter map described above clearly depends on the clutter scatterers being stationary with respect to the radar. For this reason, clutter maps have to date only been successfully applied to radar systems operating at fixed locations. A radar mounted on a moving platform, such as a ship or aircraft, will observe clutter which is moving relative to itself. Clutter objects will then move from one clutter map cell to another. Such scatterers will be reported as detections whenever they move into a new clutter map cell which has a low stored background value. Eventually, if the scatterer remains in the cell, the background in the new cell will be raised to prevent this, but the adaption may require a few scans. Also, the cell the scatterer was previously in will be left for a number of scans with an unnecessarily high threshold, which may cause wanted targets to be suppressed. If the radar motion is sufficiently fast, such problems will occur continuously, causing many false detections and loss of target detections.